1. Field of the Invention
This invention relates generally to a method for fabricating a varactor suspended over a suspended membrane and, more particularly, a method for micro-machining a varactor filter including a varactor suspended over a suspended membrane on a semiconductor substrate.
2. Discussion of the Related Art
While communications systems and radar are advancing to higher frequencies of operation, there is a clear need to allow propagation of multiple channels in relatively narrow frequency bands. This can be accomplished by large waveguide filter banks. However, such filter banks are difficult to deploy in space and are not ideal for error or ground based operations. What would help advance high frequency applications are affordable, tunable filters that allow for operation over a wide frequency bandwidth. Silicon micro-machining is an ideal approach for developing tunable filters for high frequency applications by combining high-Q resonant structures with mature RF micro-electromechanical switches (MEMS) varactor technologies.
Due to low-loss characteristics, micro-machined transmission line resonators provide an excellent method to realize high-Q filters. The microstrip line can be suspended on a thin dielectric membrane that consists of 3000/4000 Åof SiO2/SixNy layers deposited on a thermally grown 7000 Å SiO2 layer. This membrane can be formed by completely etching silicon after patterning the microstrip conductor. Cavities are formed on top and bottom silicon wafers and are metallized. These metallized cavities provide microstrip ground planes and shields when the three wafers are assembled together. The complete elimination of the substrate reduces the effective permittivity (∈eff) nearly to that of air (∈r=1), therefore minimizing the dielectric loss and enabling the high-Q characteristics. The resonator discussed herein can be fabricated by deep reactive ion etching (DRIE) processes that allow precise control of the shape and size of the etched structures. The RF performance of the resonator depends on the geometric characteristics of the etched structures, and therefore avoiding the shortcomings of wet anisotropic etching, undercutting of convex corners, pyramidal shape of etched structures, etc. is advantageous.
Important parameters for designing such a microstrip resonator include that the conductor width and the ground plane distance have a direct effect on the characteristic impedance Z0 of the line and the quality factor Qu of the resonator, provided that the shield height and the side wall distances are large enough. First, for the resonator to be loaded with MEMs varactors, the conductor width can be 600 μm. Then, to minimize their effects, the shield height can be 920 μm, and the sidewall distance from the center can be 1.8 mn. The quality factor Qu can be calculated as
      Q    u    =            π                        λ          g                ⁢        x              =          β              2        ∝            where, γ=∝+jβ is the complex propagation constant and λg is the guided wavelength.
In most cases, numerical analysis tools over-estimate the quality factor Q. This is due to various factors that cannot be modeled accurately in numerical simulations. For example, surface roughness of the metal layers in cavity walls has an important factor on the overall loss, but is not considered in the simulations.
One design challenge in transitioning a co-planar waveguide (CPW) to a microstrip line is the transition of the ground plane to a different layer. For the case of a resonator, this transition occurs with the use of two rectangular metallized posts. The formation of a rectangular post inside a silicon cavity has an intrinsic difficulty because the post is a combination of four convex corners. A convex corner is defined as the corner bounded by the fastest etching crystal planes in the silicon. The etching of rectangular convex corners in anisotropic etching solutions by KOH or TMAH leads to a deformation of the edges due to cornering undercutting. It is clear that by using wet anisotropic etching it is very difficult to control the shape and size of the final etched structure. However, a DRIE process allows control of the etched structures at an expense of the surface roughness. By utilizing DRIE in contrast to anisotropic etching, very accurate membrane suspended filters can be fabricated whose measured response very closely matches the theoretical expectations.